Magnetic recording medium

ABSTRACT

A magnetic recording medium comprising a non-magnetic support, a magnetic layer on one surface of said non-magnetic support, a back coating layer on the other surface of said non-magnetic support and a primer layer containing acicular hexagonal ferrite magnetic powder between said non-magnetic support and at least one of said magnetic layer and said back coating layer, which is used with a video recorder fitted for high density recording and in particular, which copes with prolongation of a recording time.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a magnetic recording medium which isused with a video recorder fitted for a high density recording, inparticular a thin type magnetic recording medium which copes with theprolongation of a recording time. More particularly, the presentinvention relates to a magnetic recording medium comprising anon-magnetic support, a magnetic layer on one surface of thenon-magnetic support, a back coating layer on the other surface of thenon-magnetic support and a primer layer containing acicular(needle-form) hexagonal ferrite magnetic powder between the non-magneticsupport and at least one of the magnetic layer and the back coatinglayer.

2. Description of the Related Art

Recently, with the increase of requirements on dezitization and a highrecording density such as a large recording capacity, diminishing of awavelength of a recording signal and decrease of a total tape thicknessdue to increase of recorded informations are highly desired with amagnetic recording medium, in particular a magnetic recording tape. But,decrease of the total thickness of the magnetic recording tapedeteriorates running properties and durability of the tape considerablyand is disadvantageous for the electromagnetic conversioncharacteristics.

Such phenomena are induced by large decrease of rigidity of the tapewhich is caused by the decrease of the total tape thickness. A reasonwhy the decrease of the tape rigidity influences the running propertiesand durability of the tape and also the electromagnetic conversioncharacteristics will be explained by making reference to a video tape.

The video tape runs while being wound around various types of tape guideposts of a VTR equipment at certain angles.

The tape guide posts include an upper side regulating one and a lowerside regulating one for regulating a position of the running tape. Whenthe running tape tends to depart from a position regulating post, aforce for inhibiting the departure is generated by rigidity of the tapeitself, namely a resiliency against an external force which will deformthe tape. The tape rigidity acts as a force which controls contactingproperties between the tape on which a constant tension is appliedduring running and a magnetic head, namely a spacing between the tapesurface and the magnetic head and contacting stability between them.

Since the above inhibiting force is weakened by the decrease of the taperigidity caused by the reduction of the total tape thickness, the tapeis folded or the tape edges are deformed in a wave form, and further,among the electromagnetic conversion characteristics, readout outputdecreases and output variation increases. As a method for suppressingthe decrease of the rigidity of the tape which causes a problem in thethin video tape having, for example, a thickness of 12 μm in the VHSformat and 10 μm in an 8 mm video format, an increase in the rigidity ofeach of the non-magnetic support, the magnetic recording layer and theback coating layer has been studied.

To increase the rigidity of the non-magnetic support, it has beenstudied to use a highly rigid high heat resistant material such as anaromatic polyamide and an aromatic polyimide in place of polyethyleneterephthalate (hereinafter referred to as "PET") or polyethylenenaphthalate (hereinafter referred to as "PEN") which is conventionallyused as the non-magnetic support (see Japanese Patent Publication Nos.3121/1974 and 43364/1989).

While the rigidity of the proposed materials is high, not only are thematerials are more expensive than PET or PEN, but their adhesion withthe magnetic layer or the back coating layer is poor and they have highmoisture absorbance.

Another method for increasing the rigidity of the tape includes theformation of a thin metal layer having high rigidity between themagnetic layer and the non-magnetic support, which is practically used(see Japanese Patent Kokai Publication Nos. 143433/1985, 205821/1985 and29318/1988).

Since, in the above methods, the adhesion of the magnetic layer or theback coating layer to the thin metal layer greatly decreases or there isa difference in rigidity between the layers, some problems arise, forexample, in a tape slitting step.

To increase the rigidity of the magnetic layer or the back coatinglayer, it has been proposed to increase the content of a filler such asmagnetic powder in the magnetic layer, add a reinforcing filler to thelayer, or increase the glass transition temperature of a binder resin(see Japanese Patent Publication Nos. 29334/1988 and 26285/1990 andJapanese Patent Kokai Publication No. 278020/1986).

Increase of the content of the magnetic powder in the magnetic layerworsens the durability of the tape. The addition of the reinforcingfiller does not improve dispersibility or orientation of the filler butspoils surface smoothness. When the glass transition temperature of thebinder resin is increased, the rigidity of the coated layer cannot beincreased greatly. Rather, since the binder resin becomes hard, thesurface smoothening is prevented, and it is difficult to satisfy thedispersibility and reactivity with other additives in the coated layerand durability at the same time.

As explained above, in connection with the surface smoothening and thedecrease of the thickness which are required for the magnetic recordingmedium so as to comply with the increase of recording density and therecording capacity, it is very difficult to achieve satisfactory runningproperties and durability while maintaining the electromagneticconversion characteristics unchanged.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a magnetic recordingmedium which can maintain or increase strength of the medium as a wholeand stabilize medium damage and output without interfering withrecording and reading out when the recording medium is highly smoothenedand thinned to achieve the high-density-recording and the high capacity.

According to the present invention, there is provided a magneticrecording medium comprising a non-magnetic support, a magnetic slayer onone surface of said non-magnetic support, a back coating layer on theother surface of said non-magnetic support and a primer layer containingacicular hexagonal ferrite magnetic powder between said non-magneticsupport and at least one of said magnetic layer and said back coatinglayer.

Because of the provision of the primer layer containing acicularhexagonal ferrite magnetic powder, the rigidity of the whole magneticrecording medium can be increased without deteriorating theelectromagnetic conversion characteristics.

Since each particle of the acicular hexagonal ferrite magnetic powderhas an axis of easy magnetization in a direction of a minor axis of theparticle, needle directions of the powder particles are oriented in awidth direction of the medium by the orientation in any direction.Thereby, anisotropy of the rigidity disappears in the running directionof the medium, and the rigidity of the medium is made larger than thatof the non-magnetic support.

Since the acicular hexagonal ferrite magnetic powder is a ferromagneticpowder, it is possible to form a multilayer structure in a singlecoating step and particle planes can be oriented in one direction-by theapplication of a magnetic field, so that the rigidity of the medium iseasily increased and made isotropic in the running direction of themedium.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional view of a video tape of Example 1,

FIG. 2 is a cross sectional view of an example of the magnetic recordingmedium of the present invention,

FIG. 3 is a cross sectional view of another example of the magneticrecording medium of the present invention, and

FIG. 4 is a cross sectional view of a video tape of Comparative Example1.

DETAILED DESCRIPTION OF THE DRAWINGS

The present invention will be explained in detail by making reference tothe accompanying drawings.

In FIGS. 1, 2 and 3, numeral 1 stands for a magnetic recording layer, 2for a non-magnetic support, 3 for a primer layer, and 4 for a backcoating layer.

The magnetic recording medium of the present invention is characterizedin that the primer layer is provided between the non-magnetic supportand the magnetic layer or the back coating layer, or both, and containsthe acicular hexagonal ferrite magnetic powder, whereby the rigidity ofthe medium as a whole-increases and-the anisotropy of the rigidity inthe running direction of the medium disappears.

To this end, preferably the acicular hexagonal ferrite magnetic powderhas a high aspect ratio (a ratio of a major axis to a minor axis) and isfilled in the primer layer in a high density. By the acicular hexagonalferrite magnetic powder contained in the primer layer, the rigidity ofthe magnetic recording medium as a whole is improved withoutdeteriorating the electromagnetic conversion characteristics. As aresult, it is possible to provide the magnetic recording medium whichcan maintain the running properties and durability without deterioratingthe electromagnetic conversion characteristics. The acicular hexagonalferrite magnetic powder can be oriented such that the axis of easymagentization is in a direction of thickness of the magnetic recordingmedium, or in a direction of length of the magnetic recording medium.

The acicular hexagonal ferrite magnetic powder may be a ferromagneticpowder.

Preferably, the acicular hexagonal ferrite magnetic powder has a highaspect ratio. But, the aspect ratio is not limited to a specific rangeinsofar as the acicular hexagonal ferrite magnetic powder particles canbe oriented in a certain direction by the magnetic field orientation.

Preferably, the acicular hexagonal ferrite magnetic powder particles inthe primer layer are oriented by the magnetic filed orientation in aspecific direction in relation to the running direction of the medium.In particular, the needle direction of the acicular hexagonal ferritemagnetic powder is preferably oriented in-the running direction of themedium.

An amount of the acicular hexagonal ferrite magnetic powder ispreferably from 100 to 600 parts by weight per 100 parts of a binderresin in the primer layer.

A thickness of the primer layer containing the acicular hexagonalferrite magnetic powder depends on the format of the magnetic recordingmedium. Preferably, the thickness is from 0.1 to 2.5 μm. This thicknessapplies when the primer layer is provided between the non-magneticsupport and either one of the magnetic layer and the-back coating layer,or between the non-magnetic support and both of the magnetic layer andthe back coating layer.

The primer layer may be formed by coating a composition comprising theacicular hexagonal ferrite magnetic powder, the binder resin andoptionally suitable additives in a suitable solvent on one or bothsurfaces of the non-magnetic support and drying it.

The magnetic layer may be a coating type one or a thin metal layer typeone. As the magnetic material used for forming the magnetic layer, anyof the conventionally used ones can be used. Examples of the magneticmaterial are metal oxide magnetic powder such as γ-Fe₂ O₃,cobalt-containing Fe₂ O₃, cobalt-containing Fe₃ O₄, CrO₂, bariumferrite, etc.; and non-oxide magnetic metals or metal alloys such as Fe,Fe--Ni, Fe--Co, etc.

The back coating layer may be formed by a conventional method. Forexample, a back coating paint comprising a non-magnetic powder, a binderresin, a dispersant, a lubricant and a solvent is coated directly on thesurface of the non-magnetic support or the already formed primer layerand dried. If necessary, the surface of the back coating layer issmoothened or thermally treated.

Examples of the binder resin to be contained in the magnetic layer, theback coating layer and the primer layer are thermoplastic resins such aspolyvinyl chloride resin, polyurethane resin and polyester resin incombination with an isocyanate compound, UV curable or electron-raycurable resins having a double bond, or mixtures thereof.

The non-magnetic powder in the back coating layer is used to improve therunning properties, electrical conductivity, light-shielding and thelike. Examples of the non-magnetic powder are carbon black, zinc oxide,calcium carbonate, magnesium sulfate, or mixtures thereof. If necessary,aluminum oxide, chromium oxide or titanium oxide can be added.

The non-magnetic support may be made of any of conventionally used resinmaterials. Examples of the resin material are polyesters such as PET andPEN; polyolefins such as polyethylene and polypropylene; polyimide; andaromatic polyamide. Among them, polyesters are preferred in view of thebalance among the durability, rigidity and cost of the non-magneticsupport.

The magnetic layer, the back coating layer and the primer layer may beformed successively or simultaneously. The two layers which are formedon the same surface of the non-magnetic support are preferably formed inone step.

A coating composition for each of the magnetic layer, the back coatinglayer and the primer layer is prepared by kneading the components usinga known kneading machine such as a roll mill, a kneader, a doubleplanetary mixer, an agitator mill, a sand mill, a pin mill, a ball mill,a pebble mill or a combination thereof.

PREFERRED EMBODIMENTS OF THE INVENTION

The present invention will be illustrated by the following Examples, inwhich "parts" are by weight.

EXAMPLE 1

In this Example, as a magnetic powder in a magnetic layer, there wasused ferromagnetic iron oxide powder having a major axis length of 0.15μm, an aspect ratio of 8, a coercive force of 63.7 KA/m, a saturationmagnetization of 72 emu/g and a BET specific surface area of 48 m² /g.

The magnetic powder (100 parts), a polyvinyl chloride resin (8 parts, apolyurethane resin (8 parts), carbon black (2 parts), alumina (7 parts),myristic acid (1 part), butyl stearate (2 parts), a mixed solvent ofmethyl ethyl ketone (100 parts), methyl isobutyl ketone (50 parts),toluene (100 parts) and cyclohexanone (50 parts), and a hardener (4parts) were kneaded in a pressure kneader for 2 hours, and then themixture was diluted with the solvent to adjust a viscosity suitable forsand milling and dispersed in a sand mill to obtain a dispersion.

Thereafter, a polyisocyanate compound (Colonate L manufactured by NipponPolyurethane Co., Ltd.) (4 parts) was added to the dispersion and whichwas then through a filter having an average pore size of 1 μm to obtaina magnetic paint (A) for the magnetic layer.

A primer layer paint (B-1) was prepared in the same manner as aboveexcept that an acicular hexagonal ferrite magnetic powder having a platediameter of 0.25 μm, an aspect ratio of 10 and a BET specific surfacearea of 25 m² /g was used in place of the ferromagnetic iron oxidepowder.

A back coating layer paint (B-2) was prepared by mixing and dispersingcarbon black having an average primary particle size of 20 μm (100parts), a Co-doped α-Al₂ O₃ (3 parts), a polyurethane resin (45 parts),a nitrocellulose resin (45 parts) in a mixed solvent (methyl ethylketone/toluene/cyclohexanone=2/2/1 by weight) (600 parts) in a ball milland adding a polyisocyanate compound (Colonate L manufactured by NipponPolyurethane Co., Ltd.) (10 parts), followed by filtration through afilter having an average pore size of 2 μm.

The magnetic layer paint (A) was coated on one surface of a PET filmhaving a thickness of 7 μm, oriented by a magnetic field and dried,followed by planishing by super calendering to obtain a raw-film rollhaving a magnetic layer of 2.5 μm in thickness.

On the other surface of the raw film roll, the primer layer paint (B-1)and the back coating layer paint (B-2) were simultaneously coated with acoater having two die nozzles, oriented by a magnetic field verticallyto the film plane and dried, followed by planishing by super calenderingto obtain a magnetic tape film having a total thickness of 12 μm [(A):2.5 μm, (B-1): 2.0 μm, (B-2): 0.5 μm].

The magnetic tape film was slit to a width of a half inch to obtain asample video tape.

EXAMPLE 2

The same magnetic paint (A), primer layer paint (B-1) and back coatinglayer paint (B-2) as used in Example 1 were used.

On one surface of a PET film having a thickness of 7μm, the paints (A)and (B-1) were simultaneously coated with a coater having two dienozzles which were arranged in series, oriented, dried, calendered andcured. Thereafter, on the other surface of the PET film, the paints(B-1) and (B-2) were coated in the same way as above, oriented, driedand planished by super calendering to obtain a magnetic tape film havinga total thickness of 12 μm [(A): 2.5 μm, (B-1): 1.0 μm, non-magneticsupport: 7 μm, (B-1): 1.0 μm, (B-2): 0.5 μm].

The magnetic tape film was slit-to a width of a half. inch to obtain asample video tape.

EXAMPLE 3

The same magnetic paint (A), primer layer paint (B-1) and back coatinglayer paint (B-2) as used in Example 1 were used.

On one surface of a PET film having a thickness of 7 μm, the paints (A)and (B-1) were simultaneously coated with a coater having two dienozzles which were arranged in series, oriented, dried, calendered andcured. Thereafter, on the other surface of the PET film, the backcoating paint (B-2) was coated in the same way as above, oriented, driedand planished by super calendering to obtain a magnetic tape film havinga total thickness of 12 μm [(A): 2.5 μm, (B-1): 2.0 μm, (B-2): 0.5 μm].

The magnetic tape film was slit to a width of a half inch to obtain asample video tape.

EXAMPLE 4

In the same;manner as in Example 1 except that acicular barium ferritemagnetic powder having an average particle size of 1 μm was used in theprimer layer primer (B-1) in place of the acicular hexagonal ferritemagnetic powder, a magnetic tape film having a total thickness of 12 μm[(A): 2.5 μm, (B-1): 2.0 μm, (B-2): 0.5 μm] was obtained.

The magnetic tape film was slit to a width of a half inch to obtain asample video tape.

Comparative Example 1

The same magnetic paint (A) as used in Example 1 was coated on onesurface of a PET film having a thickness of 9 μm, oriented by a magneticfield, dried and planished by super calendering to obtain a raw filmroll having a magnetic layer of 2.5 μm in thickness. Thereafter, on theother surface of the raw film roll, the same back coating paint (B-1) asused in Example 1 was coated with a die nozzle coater, dried and heattreated to obtain a magnetic tape film having a total thickness of 12 μm[(A): 2.5 μm, (B-2): 0.5 μm].

The magnetic tape film was slit to a width of a half inch to obtain asample video tape.

Comparative Example 2

In the same manner as in Comparative Example 1 except that a PEN filmwas used in place of the PET film, a sample video tape was produced.

Comparative Example 3

In the same manner as in Example 1 except that a plate-form bariumferrite magnetic powder having an average plate diameter of 0.3 μm wasused in the primer layer paint (B-1) in place of the acicular hexagonalferrite magnetic powder, a sample video tape was produced.

Comparative Example 4

On one surface of a PET film having a thickness of 7 μm, the samemagnetic paint (A) as used in Example 1 was coated, oriented by amagnetic field and dried, followed by planishing by super calendering toobtain a raw film roll having a magnetic layer of 2.5 μm in thickness.Thereafter, on the other surface of the raw film roll, the primer layerpaint (B-1) was coated by a die nozzle coater, oriented by a magneticfield, dried and heat treated to obtain a magnetic tape film having atotal thickness of 12 μm [(A): 2.5 μm, (B-1): 2.5 μm].

The magnetic tape film was slit to a width of a half inch to obtain asample video tape.

Each of the sample video tapes produced in Examples and ComparativeExamples was evaluated as follows:

(1) Young's modulus (GPa) of the tape

Using a tensile tester (manufactured by Orientec Co., Ltd.), a tensilerigidity in a machine direction (MD) or a transverse direction (TD) ismeasured. The tensile rigidity in TD is measured with a sample having alength of 1/2 inch.

(2) Tape stiffness (mg)

Using a loop stiffness tester (manufactured by Toyo Seiki Co., Ltd.),stiffness (buckling strength) in the machine direction or transversedirection is measured.

(3) RF out put (dB) at 7 MHz

Using a commercially available S-VHS video deck, a RF output at 7 MHz ismeasured with each sample tape.

(4) Running durability and output stability

The running stability is evaluated by-running each tape on thecommercially available S-VHS video deck and observing a tape surfacecondition with eyes after 100 passes.

The output stability is evaluated in terms of an evenness(maximum/minimum) of a readout output envelope during the reading out ofthe output signals.

The results are shown in the Table.

                  TABLE                                                           ______________________________________                                               Young's  Loop                                                          Exam-  modulus  stiffness                                                                              RF    Tape    Envelope                               ple    (GPa)    (mg)     output                                                                              dura-   evenness                               No.    MD/TD    MD/TD    (dB)  bility  (%)                                    ______________________________________                                        1      17.9/17.8                                                                              135/128  +0.2  Good    85                                     2      18.9/18.7                                                                              138/129  +0.5  Good    89                                     3      17.1/16.6                                                                              140/125  +0.2  Fair    83                                     4      19.2/18.6                                                                              141/130  +0.3  Good    86                                     C. 1   --/--    82/74    -0.7  One-side                                                                              57                                                                    elongation                                                                    Edge                                                                          folding                                        C. 2   --/--    91/85    -0.5  Edge    69                                                                    folding                                        C. 3   16.9/8.92                                                                              129/76   -0.6  One-side                                                                              68                                                                    elongation                                                                    Edge                                                                          folding                                        C. 4   16.6/15.8                                                                              131/113  -0.9  One-side                                                                              48                                                                    elongation                                                                    Edge                                                                          folding                                        ______________________________________                                    

As seen from the above results, the sample tapes according to thepresent invention did not suffer from the deterioration of theelectromagnetic conversion characteristics and were excellent in runningdurability and output stability, when their total thickness was madesmall.

While the S-VHS video tapes were used in the above Example, it ispossible to apply the present invention to an 8 mm video tape, an audiotape, a magnetic tape for a computer, a floppy disc, and any othermagnetic recording media.

What is claimed is:
 1. A magnetic recording medium comprising anon-magnetic support, a magnetic layer on one surface of saidnon-magnetic support, a back coating layer on the other surface of saidnon-magnetic support and a primer layer between said non-magneticsupport and at least one of said magnetic layer and said back coatinglayer, said primer layer containing acicular hexagonal ferrite magneticpowder particles having an axis of easy magnetization in a direction ofa minor axis of the acicular particle, which axis is oriented in adirection of thickness of said magnetic recording medium or in a lengthdirection of said magnetic recording medium.
 2. The magnetic recordingmedium according to claim 1, wherein said acicular hexagonal ferritemagnetic powder is oriented such that an axis of easy magnetization isin a direction of thickness of the magnetic recording medium.
 3. Themagnetic recording medium according to claim 1, wherein said acicularhexagonal ferrite magnetic powder is oriented such that an axis of easymagnetization is in a direction of length of the magnetic recordingmedium.